Process of making a titanium carbide sheathed titanium filament

ABSTRACT

TITAANIUM FILAMENTS ARE CARBURIZED TO PRODUCE A THICK TITANIUM CARBIDE SHEATH AND A CENTRAL CORE OF TITANIUM METAL WITH A DISPERSED STRENGTTHENINGG PHASE.. THE RRESULTANT PRODUCT IS INCORPORATED INTO A METAL MATRIX TO PROVIDE A COMPOSITE OF IMPROVED STRENGTH ANDD STIFFNESS.

v Filed Oct. 2a, 1958 STRENGTH (s) lO psi April 1 7, 1973 L. R. ALLEN ETAL PROCESS OF MAKING A TITANIUM CARBIDE SHEATHED TITANIUM-'FILAMEN'T' 2Sheets-Sheet 1 V 5o, I V/ '30 2O loo 500 i000 FIG. 3

EVLA'STICY MODULUS (9103s:

A ril 17, 1973 L. R. ALLEN ET'AL PROCESS OF MAKING A TITANIUM CARBIDESHEATHED TITANIUM FILAMENT 2 Sheets-Sheet 2 I Filed Oct. 28 1968 FIG. .2

United States Patent US. Cl. 148-205 4 Claims ABSTRACT OF THE DISCLOSURETitanium filaments are carburized to produce a thick titanium filamentsare carburized to produce a thick titanium carbide sheath and 0 centralcore of titanium metal with a dispersed strengthening phase. Theresultant product is incorporated into a metal matrix to provide acomposite of improved strength and stiffness.

The present invention relates to filament reinforced metal matrixcomposites of higher stillness and filament reinforcements for usetherein.

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ticity) of a matrix material but will be less reactive with the matrixthan the above filamentary reinforcement materials. Titanium carbide,commonly used in powder metallurgy composites for cutting tools, would'be a good candidate in theory in view of its known chemical inertnessand known approximate high strength and stiffness.

The following Tables I and II illustrate on a conservative basis theimprovement obtainable through titanium carbide reinforcement of metalmatrix materials in comparison with high strength alloy variations ofthe same metals. The values given for the proposed compositeproperties--strength in Table I and stilfness (modulus of elasticity) inTable II-are based on a law of mixtures analysis, an assumption of aroom temperature stillness of 50x10 psi. for titanium carbide in a formobtain able for use in a composite and known temperature of thereinforcing titanium carbide and matrix metals. The properties of thematrix metals and alloys are taken from handbook data. Densities arealso tabulated in similar fashion. The strength and modulus of titaniumcarbide alone are used in calculating composite properties; thecontribution of the matrix is ignored.

TABLE L-STRENGTH (10 P.S.I.) AT VARYING TEMPERATURES Material DensityRT. 500 F. 1,000 F. 1,500 F. 2,000 F Aluminum composite (30% TiO) 121100 85 80 Aluminum alloy (7075) 103 85 3 Titanium composite TiC) 172 140125 100 85 Titanium alloy (Ti-GAMV) 21 180 94. 1 12. 5

Nickel composite (30% TiC) 280 17 166 150 135 Nickel alloy (Rene 41) 298250 180 80 TABLE IL-STIFFNESS (10 P.S.I.) .AI VARYING TEMPERATURESMaterial RT. 500 F. 1,000 F. 1,500 F. 2,000 F.

Aluminum composite (30% T10) 20 17 Aluminum alloy (7075) 10. 7 7. 2

Titanium composite (30% T10) Titanium alloy (TLGAMV) Nickel composite(30% T10) Nickel alloy (Rene 41) NOTE .-R.T.= Room Temperature.

BACKGROUND The art of structural composite materials has developedseveral plastic-matrix and metal-matrix composite materials utilizingfilamentary reinforcing materials of high strength and modulus ofelasticity including boron, carbon and silicon carbide and siliconcarbide coated carbon. This effort is related to prior developments inregard to glass fiber reinforced composites, dispersion strengthenedalloys, filled plastics, ceramics and powder metallurgy. Considerabledifficulty remains in effectively utilizing the high modulus reinforcingfilamentary materials particularly at high temperatures which may beencountered in forming a composite or in service use (e.g. as turbineblades).

It is desirable to develop a reinforcement material which will increasethe strength and stillness (modulus of elas- It does not appear thatprior art approaches to utilization of titanium carbide have beensuccessful. This is not surprising since titanium carbide is extremelybrittle. Published reports of experiments conducted under sponsorship ofthe US. Air Force Materials Laboratory cite attempts to produce titaniumcarbide by decomposition of a carbon vapor source and diffusion ofcarbon into a titanium wire. These are Defense Documentation Center(DDC) publication AD 611757 (circa 1965) pp. 14-15 (Davies et a1.authors) and AFMLTR-67-39l (February 1968) pp. -91 (General TechnologiesCorporation, author). Additional background information on titaniumcarbide properties and preparation and potential use of such materialsand other ceramics in composites is given in NASA Technical TranslationF-102 (June 1962) pp. 74-93 (Samsonov et al., authors); Schwarzkopt,Refractory Hard Metals (MacMillan, New York, 1953); ASTIA (DDC) DocumentADl31071 (1957) (Kieffer et al.); American Society for Testing andMaterials, The Technical Potential for Metal Matrix Composites (1967)(Burte et al.); AFML-Tr-66-52 Engineering Properties of Ceramics (June1966) (Lynch et al.); NASA SP- 5055 (August 1966) Non-Glassy InorganicFibers and Composites (Harman).

OBJECTS The general objects of the present invention are to provide animprovement in filament-reinforced composite materials, and methods ofmaking the same, and particularly in reinforced metal matrix composites,which overcomes the prior art problems of high temperature oxidationand/ or reaction between reinforcement and matrix and which affords ahigh degree of increased composite strength.

Further and specific objects of the invention are to provide filamentarytitanium carbide reinforcing materials, and a method of making the same,which provides increased composite stiffness and is reliably andeconomically manufactured consistent with the foregoing objects.

SUMMARY Generally, the objects of the invention are achieved byproduction of a carburized titanium filament which is used in a metalmatrix at high temperatures. The term filament as used herein includeselongated wires, ribbons and sheets and equivalents and filament radiusmeans wire radius or half-thickness of ribbon or the like. The termtitanium includes the element titanium and higher strength alloys oftitanium.

The filament product has a characteristic core and sheath incrosssection. The core comprises the characteristic feathery transformedbeta structure of titanium normally obtained by heating the metal abovethe alpha beta transformation temperature (882 C.) and quenching with adispersed additional phase therein including particles and plates oftitanium carbide. Strengthening is obtained from both the titaniumcarbide dispersed phase and the matrix phase of transformed titaniumstructure. The sheath comprises the inert refractory material-titaniumcarbide. The wire product is made by decomposition of a carbon compoundvapor, such as methane or acetylene or application of other source ofcarbon vapor, to the surface of the titanium filament and diffusion ofthe carbon into the wire. The reaction is controlled to preventformation of a pyrolytic graphite coating, or hydrogen em'brittlement ofthe filament. The reaction is conducted with the rate of supply ofcarbon vapor to the reaction zone being limited so that it is notsubstantially greater than the rate of diffusion of carbon into thefilament and the removal of hydrogen from the reaction zone to achievethis control. Removal of hydrogen and carbon diffusion is enhanced bythe removal of titanium oxides during the reaction. Carbon containinggas is held to a pressure less than mm. Hg in the reaction zone andpreferably less than 1 mm. The reaction temperature is about 1000 C.below which reaction rate is unduly slow in essentially all significantinstances, and below the titanium-titanium carbide eutectic point whichis at about 165 C. The range is limited to betwen 1200 C. and 1650 C.for larger filaments (e.g. in wire form with radius greater than .005inch).

The finished product is incorporated into a metal matrix by conventionalcompositing techniques to produce a composite which offers substantialimprovement in strength and stiffness particularly at high temperature.A matrix of plastic can be impregnated into a bundle of reinforcement inliquid form filaments and cured to solid. A metal matrix can be cast orextruded or hot pressed. One particularly desirable form of compositeformation is to stack alternating layers of filaments and matrix metalfoil. Within the filament layers, filaments of reinforcing materialalternate with filaments of matrix metal (e.g. aluminum wires). Theentire stacked assembly is hot pressed to form the composite. Otherconventional compositing techniques now utilized with glass, boron,silicon carbide and carbon fibers and whisker reinforcements can also beutilized for making new composites containing titanium carbide. The newcomposites are characterized by a higher degree of protection frommatrix-reinforcement reaction compared to the prior art composites.

Other objects, features and advantages of the invention will be apparentto those skilled in the art from the above general description and thefollowing specific description which includes reference to theaccompanying drawings wherein:

FIG. 1 is a schematic cross-section representation of the filamentproduct.

FIG. 2 is a diagram of the carbonizing apparatus used in practicing themethod of the invention.

FIG. 3 is a curve of strengthening and stiffening effects as a functionof reaction time under a given set of pressure and temperatureconditions.

FIG. 1 shows a cross-section of the carburized titanium filament 10. Itcomprises an outer sheath 12 of completely formed near-stoichiometrictitanium carbide and an inner core 14 of titanium with dispersedneedle-form precipitates 16 of titanium carbide and additional particleof plate-like precipitates 18 therein. The sheath has an averagethickness of from 1 to 50% of wire radius, preferably 10 to 20%.

The impurity content of the filament as a result of the processing orselection of starting materials necessary to produce an effectivereinforcing filament, is less than ppm. of hydrogen, less than 3000 ppm.of oxygen and nitrogen combined.

FIG. 2 shows an apparatus which was used for producing carbided titaniumwires. The apparatus comprises a vacuum chamber 20 with internalelectrical contacts 22, 24 supplied with electric power, viafeedthroughs 26, 28 in the chamber wall, from an electric power source21. A titanium wire 10 to be carbided is connected to the contacts forreceiving an internal direct resistance heating current. An opticalpyrometer and sight port (not shown) are used to measure wiretemperature and power may be adjusted to reach and hold a desiredtemperature level. A weight indicated at 32 is used to hold the wirefrom upward distortion as it is heated.

For large scale production, the contacts 22, 24 may be rollers and areel of titanium wire may be wound and rewound in the chamber using suchrollers. A drive would be imparted to the wire and the drive tensionwould take the place of Weight 32 in holding the wire from buckling.

The vacuum chamber is evacuated by a vacuum pump (rotary mechanicalpump) 34 via an intermediate liquid nitrogen cooled trap 36 and controlvalves 38, 40.

The evacuated chamber is backfilled with gasses obtained from meteringinlet valves 50, 52, 54, the gasses being methane, argon and a halogencontaining gas (such as carbon tetrachloride, trichloroethylene)respectively. The argon or methane gasses are alternately fed to thevacuum chamber via a valve 56 and when methane is fed, it is preferablycombined with the halogen containing gas and the combination is fed tothe vacuum system via valve 58 and the cold trap.

The feed of methane or argon is purified by passage through a filter 60and a Dry Ice cooled trap 66. The filter comprises a quartz tube filledwith active uranium chips. A cooler 64 is provided for the reactor endsand a heater 66 is provided for its central zone. The chips are heatedto a temperature of between 750 and 850 C. in the central zone forreacting interstitial impurities in the feed gas with the uranium.

The vacuum chamber is evacuated to a pressure of less than 1 mm. Hg andbackfilled with purified argon several times. Then the methane-halogenmixture is admitted to the chamber. The wire is then heated to atemperature of between 1200 C. and 1650 C. A 25 mil diameter wire can bebrought up to temperatures in this range in 3-4 seconds and cooled below100 C. in 3-4 seconds. Between heating and cooldown the wire is held atthe selected temperature for a time which may be as low as seconds atthe highest temperatures up to 1500 seconds at the lowest temperature toproduce a carburized titanium wire as described above. The reactionconditions can also be varied by adjusting the partial pressures orproportions of the hydrocarbon and halogen gas and by adjusting totalpressure through variation of the reactive gas amounts or by addition ofinlet argon. Typically methane pressure is held at .2-.9 mm. Hg duringthe heating for reaction and is dropped to .005 mm. Hg just prior tocompletion of the heating-reaction step to remove excess hydrogen.Typical operating temperature for the heating reaction step is 1350 C.In order to monitor temperature properly, the optical pyrometercalibration must be adjusted to account for changing radiationcharacteristics as the wire surface changes from titanium to titaniumcarbide.

If the ultimate use of the wire is in a composite where strength is moreimportant than stiffness the shorter times are utilized. In any case thetime must be long enough to form the titanium carbide and short enoughto retain the titanium core for ductility. If the ultimate use of thecomposite places the highest premium on stiffness then the longer timesare preferred. The given times vary inversely with reaction zonetemperature, total pressure in the chamber and partial pressure of thehydrocarbon gas.

Reproducibility of results and to some extent reaction time, areaffected by the use of the halogen gas. @If the halogen gas is not usedat all, or in insulficient quantity, the titanium wire has a tendency toform compounds on its surface with the oxygen impurity. This oxide andoriginal surface oxide barriers which inhibit out-diffusion of oxygenand hydrogen. Retention of significant amounts of hydrogen during thereaction would produce embrittling hydrides and would interfere withformation of the desired titanium carbide. The oxide would inhibit thediffusion of carbon into the wire, the net result being a formation of alayer of pyrolytic graphite on the wire surface which would furtherinhibit gas diffusion.

The carbon tetrachloride gas should be present in a volumetric ratio tothe methane of at least 121000 and preferably 1:150. A ratio of greaterthan 1:50 is undesirable. In any case the halogen per se should bebetween .1% and 1% by volume of total reactive gas. The same proportionsapply to alternative halogen and hydrocarbon gasses.

The chloride reacts with surface titanium oxides to form volatiletitanium chlorides which move from the reaction zone to cooler walls ofthe reactor. Other gaseous products may be carbon monoxide and water. Avery ef fective oxygen stripping is obtained. The present process wouldappear to degrade the filament by removing oxygen content Which normallyenhances strength. But the net result is an increase of strength.

The result of chlorine addition is that the oxide barrier is dissipatedduring the reaction and that a net drop in hydrogen content of the wireoccurs during the reaction.

EXAMPLES (a) A series of 25 mil diameter wires was carburized utilizingthe apparatus and procedures described above.

. The reaction temperature was 135 0 C. and the hydrocarbon gas wasmethane, with one percent (by volume) of carbon tetrachloride, which wasat a pressure of .6 mm. Hg prior to and throughout the heating of thewires for diffusion reaction except just prior (1-2 seconds) to thetermination of heating, when pressure was reduced to .005 mm. Hg. Theheating times were varied from 50 to 1000 seconds. The resultant sampleswere tested for strength and elastic modulus. Cross section samples werecut and microscopically examined.

The curve of FIG. 3 shows the variation of strength and stiffness underthese conditions. Considerable enhancement of the strength of thetitanium filament was achieved in those runs which had shorter heatingtimes with maximum enhancement at about seconds. This strengtheningcorresponded to the formation of thin sheaths of titanium carbide on thefilaments which were within 10 to 20 percent of filament radius.Significant enhancement of modulus of elasticity was also obtained inthe same samples as indicated by the FIG. 3 curve.

(b) A series of 4 mil diameter wires was carburized utilizing theapparatus and procedures described above. The reaction temperature was1350 C. and the hydrocarbon gas was methane, with one percent (byvolume) of carbon tetrachloride which was at a pressure of .6 mm. Hgprior to and throughout the heating of the wires for diffusion reactionexcept just prior (1 to 2 seconds) to the termination of heating, whenpressure was reduced to .005 mm. Hg. The heating times were varied from15 to 60 seconds. Several resultant samples were tested for strength andelastic modulus. Fixture problems prevented readings in most cases, butin some instances valid readings demonstrating enhanced strength andmodulus were realized. One sample (which had a reaction time of 30seconds) exhibited 250,000 p.s.i. strength but could not be tested athigher stresses because of going off-scale on the tensile testers loadcell. The modulus of the sample was 46.6 10 p.s.i. Examination of afracture cross-section of the sample showed that conversion to titaniumcarbide was almost complete.

Several variations can be made Within the scope of the presentinvention. Filament strength can be enhanced by selection of highstrength titanium alloys, smooth drawn filament wires, or a polishingpre-treatment of the wire by mechanical chemical and/or electrical meansprior to carburizing reaction. The polishing can be done just prior tothe reaction, for instance, by passing a filament through a pre-treatingchamber Where it is exposed to hot chlorine containing vapors prior toentry into the reaction chamber. vPost-treatments to further enhance theinertness of the carburized surface may be performed. These incudenitriding or siliciding the outer surface of the titanium carbide sheathof the filament or depositing a layer of pyrolytic graphite over thetitanium carbide to act as a diffusion barrier to metal matrix materialsand to provide a reservoir of carbon to compensate for minor inwarddiffusion of carbon which may occur in the finished product underextreme conditions of high temperature service.

The scale-up of the process of the above examples from batch treatmentto continuous or semi-continuous treatment of filament should include astep equivalent to the pump down prior to cooling of the above examplesto prevent re-entry of hydrogen. Such equivalent steps would include acontinuous passage of the carbon containing gas over a moving titaniumfilament in a counterflow arrangement, passage of the wire from areaction chamber to an evacuated or inert gas flushed chamber forcooling or coating the filament prior to cooldown with a protectivelayer. Once the filament is cooled down to 800 C. the danger ofembrittlement is substantially passed.

Still further variations will be apparent to those skilled in the artonce given the benefit of the present disclosure. Accordingly it isintended that the above specification and accompanying drawings shall beread as illustrative and not in a limiting sense.

What is claimed is:

1. Method of making high strength, high elastic modulus titanium carbidesheathed, titanium filament comprising the step of reacting at hightemperature a titanium filament with carbon-containing vapor to producea titanium carbide sheath and a beta titanium core while evolvinghydrogen from the titanium and clearing the hydrogen from the reactionzone prior to cooling and wherein the reaction is conducted in thepresence of a subatmospheric pressure of a carbon-containing vapor andin the presence of halogen catalyst.

7 '2. The method of claim 1 wherein the filament is directly heated to atemperature above 1000 C. and below the titanium-titanium carbideeutectic temperature for a sufiiciently long time to produce a titaniumcarbide filament sheath which has a thickness of 10-20% of filament 5radius.

3. The method of claim 2 wherein the pressure of car bun-containing gasis less than 10 mm. Hg.

4. The method of claim 3 wherein the volume fraction of halogen in thecarbon-containing gas is between V1000 and /50.

References Cited UNITED STATES PATENTS 2,865,797 12/1958 McCaWley148-20.3

OTHER REFERENCES Hanzel, R. W.: Surface Hardening Processes for Titaniumand Its Alloys, Metal Progress, March 1954, pp. 89-96.

I-IYLAND BIZOT, Primary Examiner 10 G. K. WHITE, Assistant Examiner U.S.Cl. X.R.

